As is generally accepted, the resources for petroleum-based chemicals and for petroleum used as (fossil) fuel are limited. One presently used alternative resource is “biofuel” as obtained from biomass. Various sources of biomass may be used.
“First-generation biofuels” are biofuels made from sugar, starch, vegetable oil, or animal fats using conventional technology. Exemplary basic feedstocks for the production of first generation biofuels are seeds or grains such as wheat, which yield starch that is hydrolyzed and fermented into bioethanol, or sunflower seeds, which are pressed to yield vegetable oil that can be transformed into biodiesel. However, these feedstocks could instead enter the animal or human food chain. Therefore, first generation biofuels have been criticised for diverting food away from the human food chain, leading to food shortages and price increases.
By contrast, “second generation biofuel” can be produced sustainably by using biomass comprised of the residual non-food (i.e. non digestible) parts of current crops, such as stems, leaves, bagasse (sugarcane fiber residue), husks etc. that are left behind once the food crop has been extracted, as well as other feedstock that is not used for food purposes (non food crops), such as wood, annual plants and cereals that comprise little grain, and also industry waste such as sawdust, skins and pulp from fruit pressing, wine processing etc.
One common problem in producing second generation biofuels from biomass is the extraction of fermentable feedstock from the “woody” or fibrous biomass. In particular, the carbohydrates that can be hydrolyzed and fermented (in particular cellulose and, if present, hemicellulose) are intertwined with lignin (hence, in the following, such biomass will be referred to as lignocellulosic biomass').
Lignin is a complex heterogeneous polymer that cannot be subjected to the hydrolysis/fermentation cycle applied to the cellulose/hemicellulose. Lignin as commonly produced is not a particularly useful substance and is typically discarded or burned (generating some benefit as process heat) after separation. In a future efficient biorefinery, all major components of lignocellulose not only need to be separated but also all have to be utilized. The carbohydrates may be used as a platform for sugar based chemicals, e.g. ethanol.
Pretreatment (before hydrolysis) of the lignocellulosic material is conventionally achieved by means of steam heating, steam explosion or enzymatic pretreatment, among others.
A continuous process for treating a lignocellulosic feedstock is provided in WO 2006/128304. This method comprises pretreating the lignocellulosic feedstock under pressure in a pretreatment reactor at a pH between about 0.4 and about 2.0. A minor part of the lignin will be dissolved under acidic conditions like this, but the majority of the biomass lignin fraction from this process will not be soluble in water, and will be separated together with other insolubles.
Another method of converting lignocellulosic material is described in U.S. Pat. No. 6,423,145. A modified dilute acid method of hydrolyzing the cellulose and hemicellulose in lignocellulosic material under conditions to obtain higher overall fermentable sugar yields than is obtainable using dilute acid alone, comprising: impregnating a lignocellulosic feedstock with a mixture of an amount of aqueous solution of a dilute acid catalyst and a metal salt catalyst sufficient to provide higher overall fermentable sugar yields than is obtainable when hydrolyzing with dilute acid alone; loading the impregnated lignocellulosic feed-stock into a reactor and heating for a sufficient period of time to hydrolyze substantially all of the hemicellulose and greater than 45% of the cellulose to water soluble sugars; and recovering the water soluble sugars. This process produces insoluble lignins that can be separated together with non-hydrolysed biomass and other insolubles.
A more recent method of pretreatment is described in US 2008/0190013. US '013 disclosed a method for converting lignocellulosic material into biofuel. In particular embodiments, the method comprises pre-treating lignocellulosic material by dissolving the material in ionic liquids. The pretreated lignocellulosic material can be isolated, such as by precipitation with a regenerating solvent (e.g., water), and be used directly in the formation of biofuel, including undergoing hydrolysis to form sugar and fermentation to form fuel, such as bioethanol. The ionic liquid can be recycled for further use, such as by evaporation of the water introduced during precipitation, and the recycling provides a route to a hemicellulose rich fraction and an ionic liquid of consistent quality and wood dissolution characteristics. The recovered hemicelluloses are of significant utilization potential toward commodity and specialty applications. This process also produces lignins insoluble in water.
Swedish Patent no. 527 646 proposes a process for the production of fuels for engines and fuel cells from lignocellulosic material. The lignin is dissolved from the lignocellulosic material by a cook, preferably by a soda cook. The cooking liquor is gasified to produce syngas and subsequently methanol, DME etc., while the cellulose and the hemicellulose in the pulp are hydrolysed by acid (weak or strong) or enzymes and then fermented to ethanol.
An article by J. Y. Zhu et al. (“Sulfite Pretreatment (SPORL) for robust enzymatic saccharification of spruce and red pine”; Bioresource Technology 100 (2009) 2411-2418) published online on Dec. 31, 2008 reports sulfite pretreatment to overcome recalcitrance of lignocellulose for the efficient bioconversion of softwoods.
In order to arrive at a process where the conversion of second generation biomass to biofuels is performed in an economic and sustainable manner, a number of challenges need to be addressed.
The lignin component is usually burned, however it is desirable to be able to convert the lignin to valuable chemicals of commercial value. However, lignins from many of the processes are impure and are poorly soluble in water which makes them hard to process further into valuable chemicals.
The lignins in biomass are known to adsorb cellulytic enzymes and thereby having an inhibiting effect on the enzymes used to hydrolyse cellulose to cellobiose and glucose. This substantially increases the amounts of enzymes needed. In addition, the complexity of the enzyme mixture needed is substantial since the cellulose fibers still are embedded in both lignin and hemicellulose. Cost of enzymes therefore is a major challenge in biomass to biofuel processes in addition to low total yield of products. Unfortunately, all known pretreatment processes leave lignins that are inhibiting these enzymes, even when reduced to low levels (below 5%).
Recycling of enzymes is also difficult since the enzymes are unspecifically bound to the lignin in the hydrolysis process step.
Another challenge of second generation bioethanol production from a commercial point of view is the low overall yield of valuable chemicals and in particular to provide valuable chemicals of a higher value than the energy value from xylan and lignin.
In light of the prior art, a process for converting lignocellulosic biomass is sought that better prepares the cellulose for hydrolysis as well as allows for a more complete use of the biomass in providing a higher yield in performance chemicals and/or biofuel.
These objects and others are solved by a process for the production of monosaccharides, sugar based chemicals or biofuels or materials together with sulfonated lignin from lignocellulosic biomass comprising at least the following steps:    (i) pretreatment of a lignocellulosic biomass in a sulfite cooking step;    (ii) separation of the pretreated lignocellulosic biomass from step (i) into            (a) a liquid “spent sulfite liquor” phase comprising 50% or more of the lignin of the lignocellulosic biomass in the form of sulfonated lignin, and into        (b) a pulp comprising 70% or more of the cellulose of the lignocellulosic biomass;            (iii) hydrolysis of the pulp (b) from step (ii) into a sugar chemistry platform comprising monosaccharides;    (iv) optionally further processing of the monosaccharides from step (iii) resulting in useful chemicals, biofuels and/or proteins; and    (v) direct conversion or further processing of the sulfonated lignin of the liquid phase (a) from step (ii) into useful chemicals and/or materials.
In a preferred embodiment and based on the type of raw lignocellulosic biomass, a mechanical treatment step (0) may be performed prior to step (i). In said mechanical treatment step, the biomass is divided into smaller pieces or particles by mechanical treatment. This step is obsolete, for example, in case of using bagasse as a raw material.
In the pretreatment step (i), the lignocellulosic biomass is cooked with a sulfite, preferably a sodium, calcium, ammonium or magnesium sulfite under acidic, neutral or basic conditions. This pretreatment step dissolves most of the lignin as lignosulfonate together with parts of the hemicellulose. This dissolved or liquid phase (pulping liquor; also known as “Spent Sulfite Liquor”, “SSL”) is the liquid SSL phase (a) of step (ii). The cellulose is left almost intact in the pulp (b) together with parts of the hemicellulose.
By treating the lignocellulosic biomass according to the process described above, a particularly efficient biorefinery platform is generated.
By specifically employing a sulfite cook as the pretreatment step in the overall process, a good separation of the carbohydrates cellulose and hemicellulose from the lignin is achieved. The resulting pulp is particularly easy to hydrolyze due to the modification during the cook, leading to reduced cost of saccharification.
The content of the residual, non-solubilized lignin in the pulp which is remaining after the inventive treatment is found to be of no significant importance for how easily the cellulose can be hydrolyzed by enzymes. This is highly surprising and different from what has been reported earlier, see Mooney C. A. et al., 1998, “The effect of the initial pore volume and lignin content on the enzymatic hydrolysis of softwood”, Biores. Technol. 64, 2, 113-119 and Lu Y. et al., 2002, “Cellulase adsorption and an evaluation of enzyme recycle during hydrolysis of steam-exploded softwood residues”, Appl. Biochem. Biotechnol. 98-100, 641-654.
Without wishing to be bound by theory, one may assume that the sulfite pretreatment alters the lignin in a way that reduces or removes its inhibitory effect and thereby makes high hydrolysis yield at a low enzyme consumption possible.
This non-inhibitory property of the residual lignin also makes it easier to recirculate the enzymes by e.g. substrate adsorption or membrane filtration and makes the use of long-living enzymes more interesting and the overall process more economical.
Furthermore, a much higher yield of marketable products is reached compared to other processes, mainly due to the isolation and utilization of a marketable lignin product, namely lignosulfonate.
By practicing the process according to the invention, more than 80% by weight of the lignocellulosic biomass feedstock can be transformed into marketable products and yields of up to 90% of the theoretical amount of fermentable sugars are obtainable. One embodiment of the integrated overall process is shown exemplarily in FIG. 1 and is described in more detail below.
Hence, the main benefits of the present process comprise:                Lignin is converted from an insoluble form to a water soluble form that facilitates easy separation of water soluble lignins with surprisingly superior properties as performance chemicals and in the production of pure lignin chemicals.        Furthermore, the cellulose in the pulp is easily degraded by enzymes as explained above and brings the enzyme consumption and costs to an acceptable level. This is believed to be due to the fact that during the sulfite cook step, the cellulose fibers are separated and not longer embedded in lignin and hemicelluloses. Also, the lignin left in the cellulose containing pulp after the sulfite pretreatment is less inhibitory to the enzymes than native lignin in the downstream processing step of enzymatic hydrolysis. This effect was completely unexpected.        Since enzymes apparently are not adsorbed irreversibly to the lignin left in the cellulose pulp exposed to the hydrolysis step, the enzymes may also be recycled. This additionally reduces the enzyme consumption and thereby the process costs.        